# std::pair<T1,T2>::pair

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< cpp‎ | utility‎ | pair

pair(); | (1) | (constexpr since C++11) (conditionally explicit since C++11)
pair( const T1& x, const T2& y ); | (2) | (conditionally explicit since C++11) (constexpr since C++14)
| (3) |
template< class U1, class U2 > pair( U1&& x, U2&& y ); |  | (since C++11) (until C++23) (constexpr since C++14) (conditionally explicit)
template< class U1 = T1, class U2 = T2 > constexpr pair( U1&& x, U2&& y ); |  | (since C++23) (conditionally explicit)
template< class U1, class U2 > constexpr pair( pair<U1, U2>& p ); | (4) | (since C++23) (conditionally explicit)
template< class U1, class U2 > pair( const pair<U1, U2>& p ); | (5) | (conditionally explicit since C++11) (constexpr since C++14)
template< class U1, class U2 > pair( pair<U1, U2>&& p ); | (6) | (constexpr since C++14) (conditionally explicit since C++11)
template< class U1, class U2 > constexpr pair( const pair<U1, U2>&& p ); | (7) | (since C++23) (conditionally explicit)
template< pair-like P > constexpr pair ( P&& u ); | (8) | (since C++23) (conditionally explicit)
template< class... Args1, class... Args2 > pair( std::piecewise_construct_t, std::tuple<Args1...> first_args, std::tuple<Args2...> second_args ); | (9) | (since C++11) (constexpr since C++20)
pair( const pair& p ) = default; | (10) |
pair( pair&& p ) = default; | (11) | (since C++11)

Constructs a new pair.

1) Default constructor. Value-initializes both elements of the pair, `first` and `second`.

This constructor participates in overload resolution if and only if std::is_default_constructible_v<T1> and std::is_default_constructible_v<T2> are both true. This constructor is explicit if and only if either `T1` or `T2` is not implicitly default-constructible. | (since C++11)

2) Initializes `first` with x and `second` with y.

This constructor participates in overload resolution if and only if std::is_copy_constructible_v<T1> and std::is_copy_constructible_v<T2> are both true. This constructor is explicit if and only if std::is_convertible_v<const T1&, T1> is false or std::is_convertible_v<const T2&, T2> is false. | (since C++11)

3) Initializes `first` with std::forward<U1>(x) and `second` with std::forward<U2>(y).
This constructor participates in overload resolution if and only if std::is_constructible_v<T1, U1> and std::is_constructible_v<T2, U2> are both true.
This constructor is explicit if and only if std::is_convertible_v<U1, T1> is false or std::is_convertible_v<U2, T2> is false.

This constructor is defined as deleted if the initialization of `first` or `second` would bind a reference to temporary object. | (since C++23)

4) Initializes `first` with p.first and `second` with p.second.
This constructor participates in overload resolution if and only if std::is_constructible_v<T1, U1&> and std::is_constructible_v<T2, U2&> are both true.
This constructor is explicit if and only if std::is_convertible_v<U1&, T1> is false or std::is_convertible_v<U2&, T2> is false.
This constructor is defined as deleted if the initialization of `first` or `second` would bind a reference to temporary object.
5) Initializes `first` with p.first and `second` with p.second.

This constructor participates in overload resolution if and only if std::is_constructible_v<T1, const U1&> and std::is_constructible_v<T2, const U2&> are both true. This constructor is explicit if and only if std::is_convertible_v<const U1&, T1> is false or std::is_convertible_v<const U2&, T2> is false. | (since C++11)

This constructor is defined as deleted if the initialization of `first` or `second` would bind a reference to temporary object. | (since C++23)

6) Initializes `first` with std::forward<U1>(p.first) and `second` with std::forward<U2>(p.second).
This constructor participates in overload resolution if and only if std::is_constructible_v<T1, U1> and std::is_constructible_v<T2, U2> are both true.
This constructor is explicit if and only if std::is_convertible_v<U1, T1> is false or std::is_convertible_v<U2, T2> is false.

This constructor is defined as deleted if the initialization of `first` or `second` would bind a reference to temporary object. | (since C++23)

7) Initializes `first` with std::forward<const U1>(p.first) and `second` with std::forward<const U2>(p.second).
This constructor participates in overload resolution if and only if std::is_constructible_v<T1, U1> and std::is_constructible_v<T2, U2> are both true.
This constructor is explicit if and only if std::is_convertible_v<const U1, T1> is false or std::is_convertible_v<const U2, T2> is false.
This constructor is defined as deleted if the initialization of `first` or `second` would bind a reference to temporary object.
8) Given u1 as std::get<0>(std::forward(u)) and u2 as std::get<1>(std::forward(u)), denote their types as `U1` and `U2` respectively. Initializes `first` with u1 and `second` with u2.
This constructor participates in overload resolution if and only if

- std::remove_cvref(P) is not a specialization of std::ranges::subrange,
- std::is_constructible_v<T1, U1> is true, and
- std::is_constructible_v<T2, U2 is true.

This constructor is explicit if and only if std::is_convertible_v<U1, T1> is false or std::is_convertible_v<U2, T2> is false.
This constructor is defined as deleted if the initialization of `first` or `second` would bind a reference to temporary object.
9) Forwards the elements of first_args to the constructor of `first` and forwards the elements of second_args to the constructor of `second`. This is the only non-default constructor that can be used to create a pair of non-copyable non-movable types. The program is ill-formed if `first` or `second` is a reference and bound to a temporary object.
10) Copy constructor is implicitly declared(until C++11)defaulted, and is constexpr if copying of both elements satisfies the requirements on constexpr functions(since C++11).
11) Move constructor is defaulted, and is constexpr if moving of both elements satisfies the requirements on constexpr functions.

### Parameters

x | - | value to initialize the first element of this pair
y | - | value to initialize the second element of this pair
p | - | pair of values used to initialize both elements of this pair
u | - | `pair-like` object of values used to initialize both elements of this pair
first_args | - | tuple of constructor arguments to initialize the first element of this pair
second_args | - | tuple of constructor arguments to initialize the second element of this pair

### Exceptions

Does not throw exceptions unless one of the specified operations (e.g. constructor of an element) throws.

### Example

Run this code

    #include <complex>
    #include <iostream>
    #include <string>
    #include <tuple>
    #include <utility>

    int main()
    {
        auto print = [](auto rem, auto const& pair)
        {
            std::cout << rem << "(" << pair.first << ", " << pair.second << ")\n";
        };

        std::pair<int, float> p1;
        print("(1) Value-initialized: ", p1);

        std::pair<int, double> p2{42, 3.1415};
        print("(2) Initialized with two values: ", p2);

        std::pair<char, int> p4{p2};
        print("(4) Implicitly converted: ", p4);

        std::pair<std::complex<double>, std::string> p6
            {std::piecewise_construct, std::forward_as_tuple(0.123, 7.7),
                std::forward_as_tuple(10, 'a')};
        print("(8) Piecewise constructed: ", p6);
    }

Possible output:

    (1) Value-initialized: (0, 0)
    (2) Initialized with two values: (42, 3.1415)
    (4) Implicitly converted: (*, 3)
    (8) Piecewise constructed: ((0.123,7.7), aaaaaaaaaa)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 265 | C++98 | the default constructor copy-initialized `first` and `second` with T1() and T2() respectively (thus required `T1` and `T2` to be CopyConstructible) | `first` and `second` are value-initialized
LWG 2510 | C++11 | the default constructor was implicit | made conditionally-explicit
N4387 | C++11 | some constructors were implicit-only, preventing some uses | constructors made conditionally-explicit

### See also

make_pair | creates a `pair` object of type, determined by the argument types (function template)
(constructor) | constructs a new `tuple` (public member function of `std::tuple<Types...>`)

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